1. Technical Field
The present invention relates to an overload and short-circuit sensing circuit and a converter protecting circuit and method. More particularly, the present invention relates to an overload and short-circuit sensing circuit and a converter protecting circuit and method capable of sensing an overcurrent at a primary side and a short-circuit state at a secondary side.
2. Description of the Related Art
Generally, a converter power supplying device, for example, a device to which an LLC switching mode power supply (SMPS) is applied, for example, a television (TV) device, or the like, performs a protecting circuit operation. The protecting circuit operation may be mainly divided into two operations. A first operation is to turn off a device when an external load increases to a predetermined level or more and again normally operate the device when the load decreases to a normal level. When the load is maintained in a high state, the device is repeatedly turned on and off until the load is in the normal state. A second operation is to protect the device when an output of the load is in a short-circuit state. An output of the device is generally transmitted as a connector output to an image board or a liquid crystal display (LCD). Here, in the case in which a short-circuit is generated between pins due to an error of a manufacturing process, a significantly large current may instantaneously flow. When a large current flows for a long period of time, since the device may be damaged, the device should be immediately turned off. At the time of operating the protecting circuit, an abnormal phenomenon such as heat generation, a change in a board color, or the like, should not be generated.
In order to accomplish the protecting circuit function as described above, an overcurrent and short-circuit sensing circuit is required. Describing an LLC resonance control circuit in detail by way of example, a transformer for a direct current (DC)-DC control and an overcurrent sensing circuit sensing a current flowing to a primary side of the transformer are required. An existing overload preventing and short-circuit controlling circuit includes a primary current sensing unit, a half wave rectifier circuit, and a resistance dividing circuit. An amount of current flowing to the primary side is sensed as an alternating current (AC) voltage by a capacitor and is half-wave rectified by a diode. Then, the half-wave rectified voltage is smoothed by a capacitor DC smoothing circuit and is then converted into a voltage capable of comparing with an internal level of a control integrated circuit (IC) by a resistor. At the time of a normal operation, a voltage of a set voltage or less of the control IC is provided to the control IC, such that the control circuit is not operated. Meanwhile, at the time of an overload, as a level rises, the control circuit is operated, and an operation of turning off the device depending on the operation of the control circuit is repeated.
According to the related art, a turn on/off time is controlled by an RC delay of a smoothing capacitor of a level setting resistor, wherein the level setting resistor has a value determined in accord with specifications of devices. Therefore, in order to increase the RC delay, a value of the smoothing capacitor should be set to be significantly large. In order to increase the value of the smoothing capacitor, an electrolytic capacitor is used instead of a multilayered ceramic capacitor mainly used for miniaturization and lightness. Therefore, disadvantages such as a printed circuit board (PCB) area increase, a cost increased, a decrease in a device life due to characteristics of the electrolytic capacitor itself, and the like, are generated. In addition, according to the related art, since a short-circuit situation should also be detected only by an overload detecting circuit, an overload current level and a short-circuit level are not independently controlled. Further, a normal level at which the protecting circuit is operated rises, such that stability of the device is deteriorated.